First I want to become Priest of another religion, I'd like to first learn the native ways of Riombara before breaking away into my own.
That's reasonable.
Also, I could have sworn that to become a Priest character you'd need to first join another religion and gain a certain rank for the Priest class to become available?
No. Any Lord can found their own religion. Founding a religion automatically makes you a member, so of course you can then be a Priest. (Which you are automatically turned into. Also, you automatically step down from all positions.)
With all the new lands becoming available, I can probably secure a Lordship here or there. It'll just take influence and visibility in the public world, which is why I'm fighting wars, giving advice, and being my ol' Priestly self.
Ah, one question. Is a Priest-Hero possible? Always did love Battle-Priests... 
The two counter each other because of the Hero's serious disability via death potential, so it sounds highly entertaining.
Priest/Hero is certainly possible, but there are no Battle-Priests. All Priests are removed from combat, and cannot command a unit at all.
